Vicinal substituents

Classic procedures for C—C double bond formation involve yS-elimination of two vicinal substituents X and Y from a C—C single bond. 

Scheme 2 Stabilization by vicinal substituents with n-donor/a-acceptor character...

The elimination of HX(X = SOCH3, SCHj) from the [M-STol]- and [MH-HSTol] cations (Tol = CHjCgH ) from 5-9 under electron impact and chemical ionization depends strongly on the spatial orientation of the vicinal substituents . The intensity of the [M — STol] ion is appreciably higher when X and Y are cis than when they are trans. The stereochemistry of the sulfoxide group is not important since 7 and 8 exhibit very similar spectra. The elimination of HSOCHj occurs more easily from 7 and 8... 

The palladium complexes are the catalysts of choice for the cycloisomerization of the bisdienes, 9 which lead to the formation of either five- or six-membered enedienes with frarct-stereorechemistry for the vicinal substituents in the newly formed ring in high yields (Scheme 102).370... 

The importance of hydrogen bonding in determining the preferred conformation of 1,2-disubstituted ethane can be appreciated by reference to the calculations of Pople and his collaborators68. Representative systems were examined and in all cases the most stable conformer was calculated to be the one involving hydrogen bonding between the two vicinal substituents of the ethane molecule. Two typical examples of such structures are shown below. 

Nonadditivities of individual effects of vicinal substituents were recognized early. Dalling and Grant (100,101) reported that upheld (i.e., negative) a-ICSs of 2.5 to 3.5 occur at signals of substituted carbon atoms in 1,2-dimethylcyclohexanes if the methyl groups are in gauche orientation to each... 

The situation is different if electronic interaction between vicinal substituents is involved. Maciel and co-workers, for example, found large deviations from SCS additivity in 1,2-substituted ethanes if the substituents -I or -C CH are involved the deviations could be accounted for by the high polarizability of the two groups (311,312). Steric, electronic, and hydrogen-bridging effects are... 

An empirical approach for estimating geminal coupling-constants has been described for a limited class of compounds. Irrespective of whether fluorine is axially or equatorially oriented, an increase ( 3 Hz) in V results upon changing the vicinal substituent from the axial (a) to the equatorial (e) orientation, whereas changing the fluorine atom from equatorial to axial has little or no effect. An exception to the latter occurs in cases where the substituent is a ring-oxygen or -sulfur atom. 

A good diastereocontrol is obtained for the debromination of Reaction (4.14) and it is attributed to the bulky reducing agent, which approaches the radical intermediate from the less hindered face anti to the two vicinal substituents [35]. 

In cyclopentane systems the situation is simpler, as, independent of the addition type concerned, the reagent is directed by a vicinal substituent to the opposite ring face (4, 5 and 6). 

The following discussion is divided into three subsections the ketone chromophore (Section 4.4,1.1.), for which configurational assignments are based on the effect of ring dissymmetry and substitution pattern on the rotatory power of the n-rt transition. For conjugated chro-mophores (Section 4.4.1.2.) it is both the helicity of the chromophore and the vicinal substituent effect that determines the rotatory power of the 71-71 transition. Finally, the versatile stereochemical method, exciton chirality method (Section 4.4.2.), is based on the chiral interaction between the electric dipoles of the allowed transitions in two or more chromophores. 

Intennoleciilar Reactions. The intermolecular version of free radical reactions of sugar-derived radicals consists mainly of addition onto suitably activated olefins, such as acrylonitrile, generally used in excess. This approach has been explored by Giese [102]. The stereochemical course of the reaction is dictated by steric effects of the vicinal substituents, as seen from the reaction of radical 71 where equatorial attack is favored over the axial with acrylonitrile (Scheme 28). Only equatorial attack is observed using... 

Opening of the azole ring with recyclization over a vicinal substituent gives access to other ring systems, e.g. purines and pteridines. The easy reductive cleavage of the N—O bond is used in the preparation of the latter systems from 7-aminofurazano[3,4-rf]pyrimidines. 

One could imagine a complete freezing of substituent -C5 rotations if vicinal substituents were tied into a ring system, somewhat similar to octahydrofluorene or related tricyclic systems (231) (see Appendix) or to the diboma-Cp sketched in 5 [except that 5 already contains chiral substituents (20)]. 

Z)-l,2-disubstituted alkenes proved to be the most difficult class. In fact, they are not osmylated efficiently with the all purpose ligands 1F/2 F. Further studies, however, led to the discovery of the indolinyl ligands 11/21 that allowed cis dihydroxylation of these alkenes in up to 80% eel0. It should be kept in mind, however, that in the case of 1,1-disubstituted alkenes and of (Z)-l,2-disubstituted alkenes, a lowering of difference in steric requirement between the two vicinal substituents inevitably means a drop in the 7t-face discrimination since the two enantiotopic alkene 7t-faces lend to become quasi-homotopic . 

Low induced diastereoselectivity has also been reported in a number of other cases68, U9,140, particularly when the stereogenic center bears only medium-sized substituents or when it is more than one bond apart from the double bond141 142 high induced diastereoselectivity was reported for systems with a strong conformational preference, for example, vinylcycloalkanes, especially vinylcyclohexanes143, with bulky vicinal substituents, such as 2. 

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